Siemens SIPROTEC 4 7UT6 Series, Руководство пользователя, Страница: 130 / 684

a tripping effect current
Ι
Trip
= |3
Ι
0
'|
and the stabilisation or restraining current
Ι
stab
= k · ( |3
Ι
0
' – 3
Ι
0
"| – |3
Ι
0
' + 3
Ι
0
"| )
k is a stabilisation factor which will be explained below, at first we assume k = 1.
Ι
Trip
produces the tripping
effect quantity,
Ι
stab
counteracts this effect.
To clarify the situation, three important operating conditions with ideal and matched measurement quantities
are considered:
•
Through-fault current on an external earth fault:
3
Ι
0
" is in phase opposition with 3
Ι
0
', and of equal magnitude, i.e 3
Ι
0
" = –3
Ι
0
'
Ι
Trip
= |3
Ι
0
'|
Ι
stab
= |3
Ι
0
' + 3
Ι
0
'| – |3
Ι
0
' – 3
Ι
0
'| = 2 · |3
Ι
0
'|
The tripping effect current (
Ι
Trip
) equals the starpoint current; the restraining quantity (
Ι
stab
) is double the
size.
•
Internal earth fault, fed only from the starpoint
In this case 3
Ι
0
" = 0
Ι
Trip
= |3
Ι
0
'|
Ι
stab
= |3
Ι
0
' – 0| – |3
Ι
0
' + 0| = 0
The tripping effect current (
Ι
Trip
) equals the starpoint current, the restraining quantity (
Ι
stab
) is zero, i.e.
full sensitivity during internal earth fault.
•
Internal earth fault, fed from the starpoint and from the system, e.g. with equal earth current magnitude:
In this case 3
Ι
0
" = 3
Ι
0
'
Ι
Trip
= |3
Ι
0
'|
Ι
stab
= |3
Ι
0
' – 3
Ι
0
'| – |3
Ι
0
' + 3
Ι
0
'| = –2 · |3
Ι
0
'|
The tripping effect (
Ι
Trip
) equals the starpoint current; the restraining quantity (
Ι
stab
) is negative and there-
fore set to zero, i.e. full sensitivity during internal earth fault.
This result shows that for an internal fault no restraint is effective since the restraining quantity is either zero
or negative. Thus, small earth current can cause tripping. In contrast, strong restraint becomes effective for
external earth faults.
Figure 2-62 shows that the restraint is the strongest when the residual current from the
phase current transformers is high (area with negative 3
Ι
0
"/3
Ι
0
'). With ideal current transformers, 3
Ι
0
" and 3
Ι
0
'
opposite and equal, i.e. 3
Ι
0
"/3
Ι
0
' = –1.
If the starpoint current transformer is designed weaker than the phase current transformers (e.g. by selection
of a smaller accuracy limit factor or by higher secondary burden), no trip will be possible under through-fault
condition even in case of severe saturation as the magnitude of 3
Ι
0
" (negative) is always higher than that of
3
Ι
0
'.
Functions
2.3 Restricted Earth Fault Protection
130 SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016